1. Field of the Invention
The present invention relates to a volume-reducing treatment method for waste, such as shredder dust, which contains chlorine. In particular, the present invention relates to a low-cost waste treatment method which thermally decomposes or burns waste, such as shredder dust, which contains chlorine at relatively low temperatures in a reducing atmosphere, thereby completely or considerably suppressing the generation of hydrogen chloride and dioxin, obtaining a combustible gas, and greatly reducing volume.
2. Description of the Related Art
In conventional treatment methods for waste articles such as discarded automobiles and waste household electric appliances, the discarded articles are dismantled and recyclable parts are collected. Then, the remainder is torn into pieces by a shredder, and valuable metals are collected. Thus, a valueless mass is left as shredder dust. In actual practice, most of the shredder dust is disposed of in a reclamation yard without being burned. The shredder dust has a quite small bulk specific gravity, and the number of scrapped automobiles and the like is increasing. Accordingly, it is becoming difficult to find space for scrap yards where shredder dust is disposed of. In addition, in view of the fact that shredder dust contains inorganic matter, such as heavy metals, and chlorine in the form of a chlorine-containing polymer, such as a vinyl chloride resin and polychloroprene rubber, environmental regulations on scrap yards are tending to become stricter.
When shredder dust is burned, a great reduction in volume is attained, thereby facilitating its disposal. However, since the shredder dust has a high heat of combustion and contains chlorine-containing polymers, burning shredder dust involves the problems of corrosion of an incinerator caused by hydrogen chloride gas generated in combustion, and of the generation of dioxin.
Various techniques of incineration treatment for shredder dust and other waste which contains chlorine-containing polymers (and as a result which contains chlorine) are known. Japanese Patent Application Laid-open (kokai) No. 5-296429 discloses a fluidized-bed combustion apparatus wherein waste is burned in a low-oxygen atmosphere in a primary fluidized-bed, unburned solid matter is burned in a secondary fluidized-bed, a combustion exhaust gas is desulfurized and dechlorinated in a tertiary fluidized-bed, and the thus desulfurized and dechlorinated combustion exhaust gas is discharged. Japanese Patent Application Laid-open (kokai) No. 54-52873 discloses an incineration treatment method for vinyl chloride resins which involves spraying a basic calcium compound of 60-mesh or smaller particles preheated to a temperature of 200.degree. to 300.degree. C., over an object to be incinerated during incineration or thermal decomposition, so as to fix hydrogen chloride by reaction therewith. Japanese Patent Application Laid-open (kokai) No. 63-6313 discloses a combustion gas treatment method which involves subjecting an object to be incinerated to incomplete combustion in a dry distillation furnace, leading combustible gas which is generated into a secondary incinerator, and spraying an alkali agent such as calcium carbonate in the secondary incinerator so as to effectively remove harmful gases such as a hydrogen chloride gas or sulfur oxides. However, the conventional techniques described above carry out high-temperature oxidizing combustion on waste or a generated gas and add an exhaust gas treatment to the combustion treatment. That is, the conventional techniques are not intended to suppress the generation of harmful gases such as hydrogen chloride and dioxin during incineration and are not satisfactory because of both higher equipment costs and treatment and insufficient utilization of resources due to a low reaction efficiency.
Various techniques of thermal decomposition at relatively low temperatures in a reducing atmosphere for treating waste containing chlorine-containing polymers (and as a result which contains chlorine) are also known. Japanese Patent Application Laid-open (kokai) No. 48-67 discloses a method of indirect thermal decomposition treatment for waste or the like in a closed retort. Japanese Patent Application Laid-open (kokai) No. 49-78774, Japanese Patent Application Laid-open (kokai) No. 48-39572, and Japanese Patent Application Laid-open (kokai) No. 6-65582 disclose a technique of dechlorination treatment of chlorine-containing polymers in waste, attained by carrying out a first-stage thermal decomposition process at relatively low temperatures in a multi-stage thermal decomposition. Further, Japanese Patent Application Laid-open (kokai) No. 7-80433 discloses a treatment method which involves thermally decomposing vinyl chloride resin-containing dust into gases, oils, water, and solid residue at a temperature of 350.degree. to 500.degree. C. with oxygen being shut out as much as possible, and washing the generated gases and treating other elements which are generated so as to recycle them. The techniques described above, however, are for the purpose of to capturing a generated hydrogen chloride gas and carrying out an exhaust gas treatment such as washing neutralization as well as a secondary combustion treatment, and do not suppress the generation of a hydrogen chloride gas and dioxin.